Conventional subsonic aircraft typically include turbofan jet engines mounted below a wing using struts or support pylons. Referring generally to FIG. 1, the turbofan engines include a fan 12 connected to a turbine assembly (including a nose cone 14) via a shaft 16 disposed generally along the engine's longitudinal axial centerline, ENGINE C/L. The shaft is driven by a gas generator (not shown). An annular nacelle (the inlet 22 of which is shown in FIG. 1) is provided and spaced about the engine to define an inlet duct 20. During operation, the generator powers the fan which draws airflow through the inlet duct 20 for generating thrust for powering the aircraft. A portion of the airflow is channeled through the generator where it is mixed with fuel and undergoes combustion for generating combustion gases which are discharged from the engine after powering, among other things, the fan.
Still referring to FIG. 1, the nacelle includes a forward inlet 22, which is generally annular, or elliptical, when viewed from the front of the engine. The forward edge of the inlet is the nacelles' leading edge L.E. When viewed in side elevation, the internal inlet area decreases in going into the engine to eventually reach a throat R after which the internal inlet area increases. This slightly convergent/divergent shape decelerates the incoming air to acceptable levels. The inlet region forward of the throat R is referred to as the inlet lip 24. The locus of forwardmost points which lay on the inlet surface are commonly called the inlet's hilite, H. The inlet upper center location is referred to as the crown 26, the inlet lower center location is referred to as the keel 28.
The centerline of the inlet, INLET C/L, is generally drooped by an angle .theta.. The droop angle .theta. is typically in the range of about 4.degree. to about 6.degree. degrees downward from the engine centerline ENGINE C/L. Having a drooped inlet improves the aerodynamic performance of the inlet during cruise, including reducing the external drag due to spillage. The intersection between the ENGINE C/L and the INLET C/L is labeled "C/L I" (also referred to as "the start of droop"). This intersection C/L I may be located at various longitudinal positions, either for or aft of the fan face 12.
In designing a nacelle, it is desirable that the nacelle be as light as possible to help reduce the overall weight of the aircraft. It is also desirable that the nacelle be as small as possible to help reduce the aerodynamic drag due to the flow of freestream airflow thereover and therethrough. Accordingly, the length, diameter, thickness, and shape of the nacelle are very important design considerations.
In addition to the above design concerns, there are environmental factors to be considered as well. In particular, engine noise originating from the front of a gas turbine engine can pose a nuisance to communities near airports. One method of attenuating engine noise is to increase the nacelle inlet length in order to increase the space available for incorporating sound-absorbing acoustic lining. An engine nacelle utilizing this method is illustrated in FIG. 1, and is labeled item 30 and is referred to below as an extended length nacelle. As shown, the conventional inlet hilite H is planar, with the leading edge at the crown 26 being located slightly forward of the leading edge at the keel 28 as measured in side elevation relative to the engine centerline, ENGINE C/L. The extended-length nacelle hilite is also planar, though extended a distance forward from the fan 12. The extended-length nacelle is not available for use on many aircraft, mainly due to a significant increase in weight caused by the additional structure.
Still referring to FIG. 1, a second method of attenuating engine noise is to use a scarfed inlet 32. Scarfed inlets include a single-planar hilite with an extended portion projecting forward from only the leading edge of the inlet keel. Therefore, when viewed in side elevation, the scarfed inlet keel is generally forward of the inlet crown relative to either the inlet centerline or the engine centerline. The scarf angle, .gamma., is generally in the range of about 5.degree. to about 20.degree. as measured from a line perpendicular to the inlet centerline. Extending the lower surface creates a barrier to partially shield communities below the airplane from noise. The scarf inlet extended keel is a more effective noise shield than the keel of the extended length inlet. This is because the extended length inlet also has an extended crown which reflects noise down to the community, thus undermining the barrier action of its extended keel. A scarfed inlet design was the subject of U.S. Pat. No. 5,058,617.
While a scarfed inlet requires approximately half the weight needed as compared with a extended-length inlet, the single-plane scarfed inlet has the disadvantage of significantly increasing the inlet drag penalty. This drag penalty occurs because the crown lip of the scarfed inlet must be increased in size so that the inlet may function without flow separation when the engine is operating in certain flight conditions, e.g., at takeoff power on the ground during static and crosswind conditions. The increased size of the scarfed inlet crown translates into a larger frontal area for the inlet and thus more drag during cruise conditions, due to more airflow spillage around the inlet lip.
The increased-length inlet and scarfed inlet are the two basic approaches known to attenuate noise from a fixed geometry nacelle inlet. A third approach is described in a technical paper by Abbott, J. M., titled Aeroacoustic Performance of a Scoop Inlet," AIAA paper No. AIAA 77-1354, October 1977. This paper discusses experimental results for a double-arcuate scarfed inlet incorporating a curved hilite profile 34. This design is commonly referred to as a "scoop" inlet or a "double arcuate" inlet. One embodiment of a scoop inlet is shown in FIG. 2. It appears this design was patented in U.S. Pat. No. 3,946,830. The results of testing this design show that scoop inlets do improve noise attenuation, but they also undesirably create two vortices within the inlet. Consequently, the inventors herein are not aware of scoop inlets in current production use.
Thus, a need yet exists for an inlet having improved noise attenuation capability. The ideal inlet would not significantly increase inlet weight. Further, the ideal inlet would have acceptable performance characteristics and would not produce undesirable flow intake effects such as vortices. The present invention is directed to fulfilling this need.